Crosstalk is a well-known phenomenon in which an electrical signal transmitted on one wire pair in a cable bundle causes interference on other pairs in the same cable. This interference by a “crosstalk disturber” can result in data errors for communications equipment using the affected pair (“crosstalk victims”), such as analog modems, ISDN adapters, and DSL modems. Various techniques are used to reduce errors resulting from crosstalk. For example, some DSL modems test the wire pair when initializing a connection, and only utilize those parts of the spectrum that have low crosstalk impairment.
This technique is of limited value when the disturber uses time domain duplexing (TDD), as explained by FIG. 1. Remote device 101 and local device 102 are modems at two ends of a DSL connection. Remote device 101 and local device 102 take turns transmitting, with only one of the transmitters active at any one time. In this example, remote device 101 transmits during period 103 and period 104, while local device 102 transmits during period 105 and period 106. Victim device 107 is another communications device using a wire pair colocated with the pair used by local device 102, and therefore a victim subject to possible crosstalk from local device 102. Local device 102 is the disturber.
In order to minimize the effect of crosstalk, victim device 107 measures impairments at the start of a connection, and adjusts spectrum usage to avoid any frequency ranges that are strongly affected by crosstalk. The effectiveness of this strategy depends on whether or not the disturber (local device 102) is actually transmitting at the same time that victim device 107 measures the line impairments. If the victim device 107 measures at time 108, this is an accurate measurement, because the measurement time 108 coincides with the period 105 when the disturber is transmitting. However, if the crosstalk victim device 107 measures at time 109, this is inaccurate because the disturber is not transmitting at this time. Since crosstalk victim device 107 has no information about when the disturber (local device 102) transmits, the effectiveness of the crosstalk avoidance strategy when used with TDD disturbers is unpredictable. A need therefore exists to address these and other shortcomings in the prior art.